It's the age old skeptical question: Why do I need flying faders? It's asked by people who don't see what the big deal is about digital consoles and think that paging through fader layers will slow them down. In a lot of cases you can still get away without needing to worry about it. Small shows need small mixers and if you can fit everything in then that's great.
It's when the advantage of having a smaller form factor and higher input count that things start to matter. The simple fact is that it usually takes less than 200 milliseconds to snap between layers on a digital console (literally the blink of an eye) and if you know your layout it's actually much less trouble to snap from page one to page two or three than to take two or even three steps from one end of a large format analog console to the other.
But what about the really small digi desks? I'm talking about the ones that cram dozens of inputs into a frame that has say eight channel faders and eight or ten in the master section. I'll use an imaginary rock show to demonstrate.
Let's take eight drum inputs, bass, two guitars, keys and four vocals. That's sixteen channels so you'll have just drums on your first page of faders, the rest of the band on page two and effects returns and music playback on the third page. Honestly even on an analog mix I just don't touch the drums that much inside the drum group. Maybe once a song I'll change the relationship between the kick in and out mics or the snare top and bottom. It's not hard to see how paging back for those isn't really a problem, especially if you've got a drum group in the center section to make over all mix changes. (On a side note, you can have that stereo drum group show up on a single fader on most digi consoles so you've still got seven left in this example.)
That leaves instruments and vocals on page two and that's where you'll stay parked most of the night. But what if you need to work on the drums and something happens while you're on that page? Well, you can press the page button and in less than half a second be on the correct page to tame that wild guitar solo, or you can route a few important channels to subs or VCAs in the master section so they're always there no matter what page you're on. I'd put a handle on the effects returns as well. So even if you're busy tweaking the high hat EQ on page one, you can still grab a guitar solo or vocal change or re-balance the effects just by reaching over to the master section.
So that sorts out a small rock show and even one of a much larger size. I feel like I could easily handle a band with twice that many inputs on the same imaginary console. So that answers that, but then the theatre people speak up and start to complain about not being able to mix when paging.
I would say a similar setup could work. Let's take a high school show with eighteen radio mics, three across the front of the stage and a couple channels of playback for sound effects. Right away I'd set up a handle for the fronts and sound cues in the master section. Then I'd make a handle for male leads and female leads as they'll probably be the best actors, have the best technique and therefore need the least mixing. That lets me page back to the bit parts and mix those on the faders while I keep track of the leads with my other hand on the master section. With the cues you can even change what actors show up on those faders.
You also probably don't want to trust that the actors are going to perform in accordance with previously recorded fader moves, but the flying faders are still useful. My procedure would be to have each scene clear the faders from the previous scene and then raise the next faders I'm going to need a couple points so I know what to grab.
Then there's the benefit of having the faders do things besides the main mix. With a graphic EQ available on every output, it's super quick to take care of those on the fader tray instead of having to turn to an outboard rack or page through a sub menu. Mixing auxes on the fader tray is a joy as well. Consoles that have good scribble strip implementation make things easy as well, especially the ones that utilize color coding.
So that's my explanation of the virtues of motorized faders in multiple layers for those of us who haven't seen the light yet. Keep an open mind you Junior Brethren of the Knob and Fader. Some day digital will make a lot of sense for what you're working on and you'll likely never look back.